Polarity independent preamplifier for sound transmitters

ABSTRACT

A preamplifier for sound transmitters includes at least one field effect transistor having a gate electrode and first and second output electrodes. An output circuit is connected across the output electrodes, a first output electrode resistor is connected between a feed voltage input terminal and the first output electrode and a second output electrode resistor is connected between a reference potential terminal and the second output electrode. The output electrode resistors have substantially the same resistance. A polarity dependent biasing circuit is connected between the feed voltage input terminal and the gate electrode, and maintains, at the gate, substantially a predetermined negative bias potential relative to the potential at the first output electrode if a potential which is negative by a predetermined amount relative to the potential at the reference potential terminal is applied to the feed voltage input terminal, and relative to the potential at the second output electrode if a potential which is positive by the predetermined amount relative to the potential at the reference potential terminal is applied to the feed voltage input terminal.

United States Patent [72] Inventors Werner Fidi Baden; Bernhard Weingartner, Vienna, both of. Austria [21] Appl. No 838,091 [22] Filed July 1,1969 [45] Patented July 27, 1971 l 73] Assignee Altustische u. Kino-Gerate Gesellschaft m.b.ll. Wien, Germany [32] Priority July 5, 1968 [33] Austria [31 1 6520/68 [54] POLARITY INDEPENDENT PREAMPLXFIER FOR SOUND TRANSMITTERS 6 Claims, 2 Drawing Figs.

[52] U.S.Cl 179/1 A, 330/35 [51] Int. Cl 03f 3/00 [50] Field ofSearch 179/1 A; 330/35, 14

[56] Reterencs Cited UNITED STATES PATENTS 3,300,585 1/1967 Reedyk 179/1 A 3,305,638 2/1967 Teachout 3,419,811 12/1968 Recklinghausen...

ABSTRACT: A preamplifier for sound transmitters includes at least one field effect transistor having a gate electrode and first and second output electrodes An output circuit is connected across the output electrodes, a first output electrode resistor is connected between a feed voltage input terminal and the first output electrode and a second output electrode resistor is connected between a reference potential terminal and the second output electrode. The output electrode resistors have substantially the same resistance. A polarity dependent biasing circuit is connected between the feed voltage input terminal and the gate electrode, and maintains, at the gate, substantially a predetermined negative bias potential relative to the potential at the first output electrode if a potential which is negative by a predetermined amount relative to the potential at the reference potential terminal is applied to the feed voltage input terminal, and relative to the potential at the second output electrode if a potential which is positive by the predetermined amount relative to the potential at the reference potential terminal is applied to the feed voltage input terminal.

llllll IIIIIIV III'IV' AAAAAAA lllllll POLARITY INDEPENDENT PREAMPLIFIER FOR SOUND TRANSMITTERS BACKGROUN D OF THE INVENTION As compared to conventional thermionic valve circuits previously employed, for example, in conjunction with condenser microphones, the use of field effect transistors involves a lower circuit expenditure and improved noise performance of a preamplifier, lower microphonics, a longer life and a lower susceptibility to trouble. As the filament circuit is eliminated, hum interference is avoided. The relatively low feed voltages required permit the use of very simple feed circuits.

For example, where a shielded two-wire cable and centertapped transformers are used, a phantom circuit may be used to supply the feed voltage to the preamplifier. In this phantom circuit, the two sound conductors are used as outgoing conductors and the shield of the cable serves as a return conductor. If a three-wire cable is available, the third conductor may be used instead of the shielding sheath.

However, difficulties are encountered in practical studio operation because the amplifiers may be provided with PNP transistors (the positive terminal of the feed voltage source is grounded) or with NPN transistors (the negative terminal of the feed voltage source is grounded), so that damage may be caused by a wrong polarity connection.

For this reason, it is highly important for a studio technician to know the polarity of the feed voltage coming from the various amplifiers, as well as the polarity of the feed voltage ofthe sound transmitter provided with a transistorized preamplifier. This complicates the work to be performed by the technician. It also involves more equipment being available than would otherwise be needed, because a selection of equipment may be required.

SUMMARY OF THE INVENTION This invention relates to preamplifiers for sound transmitters, preferably for capacitive transducers, and more particularly, to a preamplifier including at least one field effect transistor and which is bipolar with respect to the polarity of the feed voltage source.

In accordance with the invention, all of the disadvantages mentioned above are avoided and a preamplifier, which is bipolar with respect to the polarity of the feed voltage source is provided, so that it can be operated with a grounded negative terminal or with a grounded positive terminal without any necessity of a changeover. The invention is based on the concept of utilizing the symmetrical properties of a field effect transistor, and on using special circuitry to produce a gate bias which depends on the polarity of the feed voltage, so that the gate bias of the field effect transistor will be properly set irrespective of the polarity of the feed voltage supplied thereto.

An object of the invention is to provide an improved preamplifier for sound transmitters.

Another object of the invention is to provide such a preamplifier including at least one field effect transistor.

A further object ofthe invention is to provide a preamplifier which is free of disadvantages of prior art transistorized preamplifiers.

Another object of the invention is to provide a preamplifier which is bipolar with respect to the polarity ofthe feed voltage source.

A further object of the invention is to provide a preamplifier utilizing the symmetrical properties of the field effect transistor.

Another object of the invention is to provide a preamplifier in which the gate bias of the field effect preamplifier will be properly set irrespective of the polarity of the field voltage supplied thereto.

An understanding of the principles of the invention, reference is made to the following description of typical cmbodimcnts thereof as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the Drawings FIGS. 1 and 2 are schematic wiring diagrams illustrating two different exemplary embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the circuit of the preamplifier, according to the invention, in combination with a condenser microphone M. The latter is supplied with a polarizing voltage U through a series resistor R,.. The nongrounded electrode of condenser microphone M is connected by a coupling capacitor C to the gate electrode 3 of a field effect transistor. The two other, or output, electrodes of the transistor are connected by coupling capacitors C to the primary winding of an output transformer Tr. A resistor R connects electrode 2 to one terminal tu of the feed voltage source. Electrode I is connected by resistor R to the reference line. According to the invention, the two resistors R and R have at least approximately the same resistance.

A voltage divider, comprising resistors R,, R is directly connected to the feed voltage between terminal :U,, and the reference line. The tap of the voltage divider is connected by a resistor R,,., having preferably a high resistance, to gate electrode 3 to bias the latter. An auxiliary circuit, comprising a diode D in series with a resistor R is connected in parallel to resistor R of the voltage divider.

It will be understood that the provision of this auxiliary circuit renders the voltage divider, connected across the feed voltage, dependent on polarity. Depending on the polarity of the feed voltage, the same is divided by the voltage divider according to the ratio R :(R +R or the ratio R R /R +R :(R, -l-R R lR +R so that the bias of the gate electrode is changed in dependence on the mode of operation of the field effect transistor. In the first case the polarity of the feed voltage is such that diode D is blocked so that resistor R does not shunt resistor R On the other hand, if the polarity of the feed voltage permits a flow of current through the diode, resistor R will directly shunt resistor R if the resistance of the diode is neglected, so that the gate bias voltage is lower than the voltage at electrode 1. This will be the case if a positive voltage is applied to the terminal iU The field effect transistor will then be used in its normal circuit, where electrode 1 is the source and electrode 2 the drain electrode.

These conditions will be reversed ifa negative voltage is applied to terminal iU In this case, electrode 2 acts as a source electrode and electrode 1 as a drain electrode. Because a negative voltage is applied to diode D, this diode is blocked and the voltage applied to gate electrode 3 is raised, so that the bias between electrodes 3 and 2 is the same as that applied between electrodes 3 and 1 when the feed voltage had the opposite polarity.

There will be no change at the output of the preamplifier because the arrangement at the output is substantially symmetrical, as the resistors R and R are equal in value.

The circuit shown in FIG. 2 differs from that of FIG. 1 in that a fixed voltage source U,,, rather than diode D, is connected in the auxiliary circuit. Voltage source U,, may comprise, e.g., a battery. Hence, the auxiliary circuit consists of voltage source U connected in series with resistor R Resistor R is a common constituent of the auxiliary circuit and of the voltage divider circuit. The bias voltage for gate electrode 3 of the field effect transistor is developed across resistor R Depending on the polarity of feed voltage U,,, the current flowing through resistor R;, will be either increased or reduced, so that the proper bias voltage will be applied to gate electrode 3. This bias voltage is designated U in FIG. 2 and can be calculated as U R3((/|,VR1:EVU|,R2)H 1c31z1+1:21e,+ MR This formula can easily be derived if the voltage divider equation is applied to the circuit of FIG 2.

The circuit, shown in FIG. 2, of a bipolar preamplifier for sound transmitters, preferably capacitive transducers, has the same properties as the circuit shown in FIG. 1, in that the feed voltage can be applied regardless of its polarity. Hence. a preamplifier according to the invention may be supplied with feed voltage from a power amplifier or the like provided with PNP transistors or with NPN transistors, and may be connected without need for special attention, with one type being sufficient for all cases. Without the measures according to the invention, a reversal of the polarity would result in a gate bias which is so undesirable that, e.g., with barrier layer field effect transistors, the gate-diode region becomes conducting or, with insulated field effect transistors, the gate bias lies outside the utilizablc region ofthe family ofcharacteristics.

We claim:

l. in combination with a sound transmitter, a preamplifier circuit arrangement comprising at least one field effect transistor having first and second output electrodes and a gate electrode; means providing a low impedance path connecting said gate electrode said sound transmitter; a feed voltage input terminal; a reference potential terminal; an output circuit connected across said first and second output electrodes; a first output electrode resistor connected between said feed voltage input terminal and said first output electrode; a second output electrode resistor connected between said reference potential terminal and said second output electrode; said first and second output electrode resistors having substantially the same resistance; and a polarity dependent biasing circuit con nected between said feed voltage input terminal and said gate electrode, said polarity dependent biasing circuit maintaining,

at said gate electrode, substantially a predetermined negative bias potential, relative to the potential at said first output electrode if a potential which is negative by a predetermined amount relative to the potential at said reference potential ter minal is applied to said feed voltage input terminal, or relative to the potential at said second output electrode if a potential which is positive by said predetermined amount relative to the potential at said reference potential terminal is applied to said feed voltage input terminal.

2. The combination claimed in claim 1, in which said sound transmitter is a condenser microphone.

3. An apparatus as claimed in claim 1, in which said biasing circuit comprises a polarity dependent voltage divider arranged to divide a direct current voltage, applied thereacross, in different ratios in dependence on the polarity of said direct current voltage.

4. An apparatus as claimed in claim 3, in which said voltage divider comprises three resistors and a diode.

5. An apparatus as claimed in claim 1, in which said biasing circuit comprises an auxiliary voltage source; a first deriving circuit deriving first direct current voltage from said auxiliary voltage source; a second deriving circuit deriving a second circuit current voltage from the potential applied to said feed voltage input terminal; and a combining circuit superimposing said first and second direct current voltages and applying the resulting voltage to said gate electrode.

6. An apparatus as claimed in claim 5, in which said auxiliary voltage source is a battery. 

1. In combination with a sound transmitter, a preamplifier circuit arrangement comprising at least one field effect transistor having first and second output electrodes and a gate electrode; means providing a low impedance path connecting said gate electrode said sound transmitter; a feed voltage input terminal; a reference potential terminal; an output circuit connected across said first and second output electrodes; a first output electrode resistor connected between said feed voltage input terminal and said first output electrode; a second output electrode resistor connected between said reference potential terminal and said second output electrode; said first and second output electrode resistors having substantially the same resistance; and a polarity dependent biasing circuit connected between said feed voltage input terminal and said gate electrode, said polarity dependent biasing circuit maintaining, at said gate electrode, substantially a predetermined negative bias potential, relative to the potential at said first output electrode if a potential which is negative by a predetermined amount relative to the potential at said reference potenTial terminal is applied to said feed voltage input terminal, or relative to the potential at said second output electrode if a potential which is positive by said predetermined amount relative to the potential at said reference potential terminal is applied to said feed voltage input terminal.
 2. The combination claimed in claim 1, in which said sound transmitter is a condenser microphone.
 3. An apparatus as claimed in claim 1, in which said biasing circuit comprises a polarity dependent voltage divider arranged to divide a direct current voltage, applied thereacross, in different ratios in dependence on the polarity of said direct current voltage.
 4. An apparatus as claimed in claim 3, in which said voltage divider comprises three resistors and a diode.
 5. An apparatus as claimed in claim 1, in which said biasing circuit comprises an auxiliary voltage source; a first deriving circuit deriving first direct current voltage from said auxiliary voltage source; a second deriving circuit deriving a second circuit current voltage from the potential applied to said feed voltage input terminal; and a combining circuit superimposing said first and second direct current voltages and applying the resulting voltage to said gate electrode.
 6. An apparatus as claimed in claim 5, in which said auxiliary voltage source is a battery. 